The white paper “Overcoming AAV cargo limits: Dual AAV and triple vector strategies in gene therapy” describes the standard methods for increasing genetic material in an AAV-vector by combining either two or more vectors to deliver the target.

Adeno-associated virus (AAV) is widely used as an in-vivo gene-delivery platform because it is generally well-tolerated, can be engineered for different tissue tropisms, and will support long-term expression in non-dividing cells. Despite these strengths, most projects run into the same bottleneck; AAV’s limited genome packaging capacity. The practical packaging capacity of AAV is ~4.7-5.0 kb. Beyond that, vectors have an increasing percentage of truncated genomes and a higher fraction of empty capsids.

The paper focuses on how the field has addressed the packaging bottleneck with dual AAV and triple AAV strategies that split large expression cassettes across multiple vectors relying on the host cell to reassemble functional products. There are primarily three methods to reassemble the target: DNA-level recombination of overlapping genomes, mRNA trans-splicing, and protein-level reconstitution (using split inteins). All three share a common design principle; each individual vector remains within the tolerated AAV genome size, while the full-length product is reconstructed inside the target cell.

A featured collaborative case study illustrates real-world dual-vector rationale and impact: intranasal AAV9 delivery of a split CRISPR-Cas9 system targeting HTR2A (5-HT2A receptor) in mice. Because the Cas9 cassette alone consumes most of the AAV payload budget, the design uses two vectors to distribute the Cas9 components, guide RNA module, and reporter elements (Rohn et al. 2023, Rohn et al. 2024). The project achieved reduced receptor expression and measurable behavioral improvement, demonstrating real-world potential of dual-vector systems.

Finally, the paper emphasizes that quality control and validation are critical elements of vector architecture because even minor impurities or inaccuracies in a viral preparation can fundamentally alter how the vector behaves in vitro or in vivo.  Control viruses, such as Vector Biolabs’ dual AAV GFP constructs, can be used to confirm co-transduction, recombination/splicing performance, and expression behavior before advancing custom constructs.

For vector developers and partners like Vector Biolabs, the practical question is no longer whether AAV can be used, but which combination of payload sizing, split-vector architecture, and capsid design makes the most sense for a given indication. 

By overcoming AAV’s size restriction, dual and triple vector systems enable delivery of large cDNAs that were previously incompatible with AAV, opening new opportunities for gene delivery, therapeutic development, and clinical translation.

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